However, an understanding of these low-level tools can be helpful if
you want to understand git's internals.

The core git is often called "plumbing", with the prettier user
interfaces on top of it called "porcelain". You may not want to use the
plumbing directly very often, but it can be good to know what the
plumbing does for when the porcelain isn't flushing.

Note

Deeper technical details are often marked as Notes, which you can
skip on your first reading.

Creating a git repository

Creating a new git repository couldn't be easier: all git repositories start
out empty, and the only thing you need to do is find yourself a
subdirectory that you want to use as a working tree - either an empty
one for a totally new project, or an existing working tree that you want
to import into git.

For our first example, we're going to start a totally new repository from
scratch, with no pre-existing files, and we'll call it git-tutorial.
To start up, create a subdirectory for it, change into that
subdirectory, and initialize the git infrastructure with git-init:

$ mkdir git-tutorial
$ cd git-tutorial
$ git init

to which git will reply

Initialized empty Git repository in .git/

which is just git's way of saying that you haven't been doing anything
strange, and that it will have created a local .git directory setup for
your new project. You will now have a .git directory, and you can
inspect that with ls. For your new empty project, it should show you
three entries, among other things:

a file called HEAD, that has ref: refs/heads/master in it.
This is similar to a symbolic link and points at
refs/heads/master relative to the HEAD file.

Don't worry about the fact that the file that the HEAD link points to
doesn't even exist yet -- you haven't created the commit that will
start your HEAD development branch yet.

a subdirectory called objects, which will contain all the
objects of your project. You should never have any real reason to
look at the objects directly, but you might want to know that these
objects are what contains all the real data in your repository.

a subdirectory called refs, which contains references to objects.

In particular, the refs subdirectory will contain two other
subdirectories, named heads and tags respectively. They do
exactly what their names imply: they contain references to any number
of different heads of development (aka branches), and to any
tags that you have created to name specific versions in your
repository.

One note: the special master head is the default branch, which is
why the .git/HEAD file was created points to it even if it
doesn't yet exist. Basically, the HEAD link is supposed to always
point to the branch you are working on right now, and you always
start out expecting to work on the master branch.

However, this is only a convention, and you can name your branches
anything you want, and don't have to ever even have a master
branch. A number of the git tools will assume that .git/HEAD is
valid, though.

Note

An object is identified by its 160-bit SHA1 hash, aka object name,
and a reference to an object is always the 40-byte hex
representation of that SHA1 name. The files in the refs
subdirectory are expected to contain these hex references
(usually with a final {apostrophe}\n{apostrophe} at the end), and you should thus
expect to see a number of 41-byte files containing these
references in these refs subdirectories when you actually start
populating your tree.

You have now created your first git repository. Of course, since it's
empty, that's not very useful, so let's start populating it with data.

Populating a git repository

We'll keep this simple and stupid, so we'll start off with populating a
few trivial files just to get a feel for it.

Start off with just creating any random files that you want to maintain
in your git repository. We'll start off with a few bad examples, just to
get a feel for how this works:

$ echo "Hello World" >hello
$ echo "Silly example" >example

you have now created two files in your working tree (aka working directory),
but to actually check in your hard work, you will have to go through two steps:

fill in the index file (aka cache) with the information about your
working tree state.

commit that index file as an object.

The first step is trivial: when you want to tell git about any changes
to your working tree, you use the git-update-index program. That
program normally just takes a list of filenames you want to update, but
to avoid trivial mistakes, it refuses to add new entries to the index
(or remove existing ones) unless you explicitly tell it that you're
adding a new entry with the --add flag (or removing an entry with the
--remove) flag.

So to populate the index with the two files you just created, you can do

$ git update-index --add hello example

and you have now told git to track those two files.

In fact, as you did that, if you now look into your object directory,
you'll notice that git will have added two new objects to the object
database. If you did exactly the steps above, you should now be able to do

which correspond with the objects with names of 557db... and
f24c7... respectively.

If you want to, you can use git-cat-file to look at those objects, but
you'll have to use the object name, not the filename of the object:

$ git cat-file -t 557db03de997c86a4a028e1ebd3a1ceb225be238

where the -t tells git-cat-file to tell you what the "type" of the
object is. git will tell you that you have a "blob" object (i.e., just a
regular file), and you can see the contents with

$ git cat-file "blob" 557db03

which will print out "Hello World". The object 557db03 is nothing
more than the contents of your file hello.

Note

Don't confuse that object with the file hello itself. The
object is literally just those specific contents of the file, and
however much you later change the contents in file hello, the object
we just looked at will never change. Objects are immutable.

Note

The second example demonstrates that you can
abbreviate the object name to only the first several
hexadecimal digits in most places.

Anyway, as we mentioned previously, you normally never actually take a
look at the objects themselves, and typing long 40-character hex
names is not something you'd normally want to do. The above digression
was just to show that git-update-index did something magical, and
actually saved away the contents of your files into the git object
database.

Updating the index did something else too: it created a .git/index
file. This is the index that describes your current working tree, and
something you should be very aware of. Again, you normally never worry
about the index file itself, but you should be aware of the fact that
you have not actually really "checked in" your files into git so far,
you've only told git about them.

However, since git knows about them, you can now start using some of the
most basic git commands to manipulate the files or look at their status.

In particular, let's not even check in the two files into git yet, we'll
start off by adding another line to hello first:

$ echo "It's a new day for git" >>hello

and you can now, since you told git about the previous state of hello, ask
git what has changed in the tree compared to your old index, using the
git-diff-files command:

$ git diff-files

Oops. That wasn't very readable. It just spit out its own internal
version of a diff, but that internal version really just tells you
that it has noticed that "hello" has been modified, and that the old object
contents it had have been replaced with something else.

To make it readable, we can tell git-diff-files to output the
differences as a patch, using the -p flag:

Committing git state

Now, we want to go to the next stage in git, which is to take the files
that git knows about in the index, and commit them as a real tree. We do
that in two phases: creating a tree object, and committing that tree
object as a commit object together with an explanation of what the
tree was all about, along with information of how we came to that state.

Creating a tree object is trivial, and is done with git-write-tree.
There are no options or other input: git write-tree will take the
current index state, and write an object that describes that whole
index. In other words, we're now tying together all the different
filenames with their contents (and their permissions), and we're
creating the equivalent of a git "directory" object:

$ git write-tree

and this will just output the name of the resulting tree, in this case
(if you have done exactly as I've described) it should be

8988da15d077d4829fc51d8544c097def6644dbb

which is another incomprehensible object name. Again, if you want to,
you can use git cat-file -t 8988d\... to see that this time the object
is not a "blob" object, but a "tree" object (you can also use
git cat-file to actually output the raw object contents, but you'll see
mainly a binary mess, so that's less interesting).

However -- normally you'd never use git-write-tree on its own, because
normally you always commit a tree into a commit object using the
git-commit-tree command. In fact, it's easier to not actually use
git-write-tree on its own at all, but to just pass its result in as an
argument to git-commit-tree.

git-commit-tree normally takes several arguments -- it wants to know
what the parent of a commit was, but since this is the first commit
ever in this new repository, and it has no parents, we only need to pass in
the object name of the tree. However, git-commit-tree also wants to get a
commit message on its standard input, and it will write out the resulting
object name for the commit to its standard output.

And this is where we create the .git/refs/heads/master file
which is pointed at by HEAD. This file is supposed to contain
the reference to the top-of-tree of the master branch, and since
that's exactly what git-commit-tree spits out, we can do this
all with a sequence of simple shell commands:

In this case this creates a totally new commit that is not related to
anything else. Normally you do this only once for a project ever, and
all later commits will be parented on top of an earlier commit.

Again, normally you'd never actually do this by hand. There is a
helpful script called git commit that will do all of this for you. So
you could have just written git commit
instead, and it would have done the above magic scripting for you.

Making a change

Remember how we did the git-update-index on file hello and then we
changed hello afterward, and could compare the new state of hello with the
state we saved in the index file?

Further, remember how I said that git-write-tree writes the contents
of the index file to the tree, and thus what we just committed was in
fact the original contents of the file hello, not the new ones. We did
that on purpose, to show the difference between the index state, and the
state in the working tree, and how they don't have to match, even
when we commit things.

As before, if we do git diff-files -p in our git-tutorial project,
we'll still see the same difference we saw last time: the index file
hasn't changed by the act of committing anything. However, now that we
have committed something, we can also learn to use a new command:
git-diff-index.

Unlike git-diff-files, which showed the difference between the index
file and the working tree, git-diff-index shows the differences
between a committed tree and either the index file or the working
tree. In other words, git-diff-index wants a tree to be diffed
against, and before we did the commit, we couldn't do that, because we
didn't have anything to diff against.

But now we can do

$ git diff-index -p HEAD

(where -p has the same meaning as it did in git-diff-files), and it
will show us the same difference, but for a totally different reason.
Now we're comparing the working tree not against the index file,
but against the tree we just wrote. It just so happens that those two
are obviously the same, so we get the same result.

Again, because this is a common operation, you can also just shorthand
it with

$ git diff HEAD

which ends up doing the above for you.

In other words, git-diff-index normally compares a tree against the
working tree, but when given the --cached flag, it is told to
instead compare against just the index cache contents, and ignore the
current working tree state entirely. Since we just wrote the index
file to HEAD, doing git diff-index --cached -p HEAD should thus return
an empty set of differences, and that's exactly what it does.

Note

================
git-diff-index really always uses the index for its
comparisons, and saying that it compares a tree against the working
tree is thus not strictly accurate. In particular, the list of
files to compare (the "meta-data") always comes from the index file,
regardless of whether the --cached flag is used or not. The --cached
flag really only determines whether the file contents to be compared
come from the working tree or not.

This is not hard to understand, as soon as you realize that git simply
never knows (or cares) about files that it is not told about
explicitly. git will never go looking for files to compare, it
expects you to tell it what the files are, and that's what the index